The broad, long-term objective of this application is to explore the role of genetic defects in human male infertility. Millions of men who are otherwise healthy produce few or no sperm, resulting in infertility. In the great majority of such otherwise normal men, the causes of spermatogenic failure cannot be pinpointed at present. Numerous studies have addressed the possible roles of infectious agents, immune processes, varicoceles, chemical insults, or other physiologic or environmental factors. By contrast, the present application focuses on genetic factors. More specifically, this application will test the hypothesis that unidentified mutations in specific X-linked genes and Y-linked genes are significant causes of unexplained spermatogenic failure in otherwise healthy men. One thousand men with spermatogenic failure will be tested for deletions or point mutations in 30 candidate genes located on the X or Y-chromosomes.